1. Field of the Invention
The present invention relates to an optical fiber which is suitable as a transmission medium for optical communication, and in particular relates to an optical fiber having excellent high-power endurance.
2. Related Background of the Invention
At present, optical fiber communication networks are being expanded from trunk lines to ordinary homes, and are gaining wide recognition as FTTH (Fiber To The Home) services. Nearly all the optical fibers employed in such communication networks are silica-based fibers; by adding a refractive index increaser such as for example GeO2 to the core region, full-reflection optical waveguide structures having a refractive index difference, with the cladding are obtained.
For example, in Document 1: Proceedings of 2008 TRICE General Conference, B-10-24, p. 306 (Mar. 18 to 21, 2008), it is noted that when small-radius bending is applied to an optical fiber in a state in which high-power light is propagating in the optical fiber, there is the possibility that light leaking from the core due to this bending may cause combustion of the resin coating. Also, in Document 1, it is noted that rises in temperature of the resin coating depend on the amount of increase in loss arising from bending. That is, in Document 1, it is stated that for bending at the same bending radius, a fiber with smaller loss increase due to bending, such as for example a trench fiber or a HAF (Hole-Assisted fiber), is desirable.
Also, Document 2: OFC2007, JWA2 (Mar. 25, 2007) describes trench fibers with a small loss increase arising from bending. For example, referring to Table 1 in which are compared the characteristics of trench fibers and single-mode fibers (SMFs) which are standard step-index type fibers, the attenuation is seen to be greater for a trench fiber than for a standard SMF at wavelengths of both 1310 nm and 1550 nm.
Further, Document 3: “HITACHI DENSEN”, No. 26, pp. 71 to 76 (January 2007) describes a hole-assisted fiber with a small loss increase arising from bending. In this Document 3, it is stated that the propagation loss of the hole-assisted fiber is 0.50 dB/km and 0.35 dB/km at wavelengths of 1.31 μm and 1.55 μm respectively. These values are still larger than those of the trench fibers described in the above-mentioned Documents 1 and 2.